Ethnopharmacological Potential of Phytochemicals and Phytogenic Products against Human RNA Viral Diseases as Preventive Therapeutics

RNA viruses have been the most destructive due to their transmissibility and lack of control measures. Developments of vaccines for RNA viruses are very tough or almost impossible as viruses are highly mutable. For the last few decades, most of the epidemic and pandemic viral diseases have wreaked huge devastation with innumerable fatalities. To combat this threat to mankind, plant-derived novel antiviral products may contribute as reliable alternatives. They are assumed to be nontoxic, less hazardous, and safe compounds that have been in uses in the beginning of human civilization. In this growing COVID-19 pandemic, the present review amalgamates and depicts the role of various plant products in curing viral diseases in humans.


Introduction
The continuous increase in the human population has made globalization and contact between people, domestic animals, and wildlife inevitable. This increased connectivity between man and the wild has led to some devastating diseases from wildlife reservoirs with a heavy mortality toll. Some of them like the human immunodeficiency virus (HIV), H1N1 influenza, the highly pathogenic H5N1 avian influenza, Nipah virus, Hendra virus, severe acute respiratory syndrome coronavirus (SARS-CoV), Ebola virus (EBOV), and more recently, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have created havoc in the world [1,2]. Studies of decades show that the RNA viruses are the most common class of virus, which are often highlighted as the preeminent member of pathogen class that is behind new human diseases, with a rate of 2 to 3 novel viruses being exposed each year [3,4]. Meanwhile, vaccine development is introduced each time after the advent of new threats. Currently, a number of vaccines are considered to be a critical component in the prevention of viral infections [5], but most of them show side effects, and many of the viruses acquired resistance against them [6]. Therefore, the unavailability of potent vaccines increased mortality numbers. Vaccines are approximately only 50% effective; hence, there is a strong need for antiviral compounds with the ability to suppress viruses, devoid of any or any major side effects [6]. Fascinatingly, traditional medicine has been used in several parts of the worlds like in Asian, African, European, and South American countries. Because the traditional healthcare system is easily culturally acceptable and relatively cheaper compared to costly orthodox medicines [7]. Starting from past to present, wild plants are used traditionally due to having their medicinal properties in between rural communities and the societies to making a bridge in generation after generation, standing in this present scenario where the communication technology developed faster and spread data in wide distances that helps the people to enrich their traditional knowledge, which are essential in our daily life [8]. In addition, as compared to the orthodox system, plant-based natural pharmacotherapy can be used easily as proper alternatives for treating viral diseases [9]. In this review article, our main motive is to highlight the protective measures of RNA viral diseases in humans by using easily available medicinal plants, crude extracts, or active compounds.

Potential Pharmacological Targets of RNA Viruses in Host
RNA viruses contain RNA molecule to carry their genetic information. However, it also contains the information required for the synthesis of its own protein. These proteins may help in replication and spread to other susceptible host. Due to their reduced coding capacity, they are dependent on host cell to complete multiplication cycle [10]. This generalized strategy is followed by many other RNA viruses like human immunodeficiency virus (HIV) [11]. Corona virus disease 2019  caused by the RNA virus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a serious threat to mankind as well as the world economy [2]. Most interestingly, host RNAbinding protein is attached with the specific architecture of RNA. In case of SARS-CoV-2, infection cycle is started by the interaction of receptor-binding domain (RBD) with angiotensin-converting enzyme 2 (ACE2) receptors present on the cell surface of human. After binding, transmembrane protease and serine 2 cleaved S protein facilitate the entry of virus. The SARS-CoV-2 particles enter the host through either direct membrane fusion or endocytosis. After its translation to polypeptides, the main protease (M pro ) and papain-like protease (PL pro ) cleaved the translated polypeptides to release nonstructural proteins (nsps), whereas RNA-dependent-RNA polymerase (RdRp) facilitates RNA transcription and replication. On the other hand, its mRNA is translated to produce structural and accessory proteins in endoplasmic reticulum (ER). Then, structural proteins are assembled with nucleocapsid into the secretory vesicles (lumen). Finally, the assembled SARS-CoV-2 particles were released by following the exocytosis method [12].

Efficiency of RNA Viruses for Pathogenesis
Till now, 13 families of single-stranded RNA (ssRNA) and 1 family of double-stranded RNA (dsRNA) virus have been included in virus classification [13]. ssRNA viruses are further classified with having either positive or negative sense strands [14]. Those viruses are capable to modify cellular metabolism upon successful infection [15]. They invariably use host cell ribosomes for the production of protein and related enzymes [16]. Interestingly, RNA viruses have tremendous adaptability to a new environment. They can also be able to handle harsh situations related to different selection pressures encountered, though the selective pressures not only include the host's immune system and defense mechanisms but are also encountered by challenges created through the application of drugs such as protease inhibitors of hepatitis B and C virus and inhibitors of HIV-1 reversetranscriptase [1]. Due to exceptionally shorter generation times and faster evolutionary rates, in a very short-time span, only the RNA viruses are able to transmit a disease to new host species [1]. The fast evolutionary rate of RNA viruses incurred due to the rapid rate of replication errors [17]. It is justified by many studies that the rate of mutation of RNA viruses is about six times greater than that of their cellular hosts. This is remarkably due to high mutation rate, which helps to maintain a distinctive rate of adaptive evolution that is shown in Figure 1 [1]. Basic mode of infection related to other factors are depicted in Figure 2.

Plant-Based Therapies against RNA Viral Disease
Medicinal plant (MP) contains a substantial amount of pharmacologically important secondary metabolites that can be used for therapeutic purposes. Since the very past, different plant parts or extracts had been utilized by local people to prevent many diseases up till now [18]. Interestingly, viral infections with sharp mortality and morbidity rates are one of the most key concerns of human deaths worldwide [6]. In this irresistible situation of global pandemic, world research has been shifted to find out potential vaccines [4]. Due to the lack of vaccines and other standard therapies, viral diseases grow rapidly throughout the world. In this situation, discovery of the novel antiviral drugs is of utmost importance. Moreover, cost-effectiveness and high efficacy are always a matter of concern for drug discovery [19]. In many cases, the synergistic effect of plant extract  2 BioMed Research International in combinations with other compounds has shown better results against these diseases [20]. Since the very early times, active compounds were isolated and identified by following the bioassay-guided fractionation techniques that take longer time and most of the times do not show positive results. The recent methods of drug discovery are more efficient due to quick isolation and faster progression of novel drug formulation that also includes rapid clinical trials [21]. A concise strategy for natural drug discovery is shown in Figure 3.
On the other hand, commonly used natural plant products in Ayurveda are clearly mentioned in Susruta Samhita and Charaka Samhita or it can be obtained through traditional knowledges [22]. Conventionally used extracts of specific plant parts like roots, bark, stem, seeds, fruits and flowers, dietary supplements, plant derivatives (phyto-constituents), and nutraceuticals found wide applications in the treatment of wide range of diseases. Besides that, the survey report indicates that among commonly used medicines, one quarter of the compounds are isolated from plant sources [22]. Hence, the scientists working on drug discovery are trying to focus on the medicinal plants used in ethnobotany and trying to document them to establish their natural, inherent, and positive effects against specific diseases [7,23]. But in comparison with the investigation of antimicrobial properties, the investigation of plant-derived antiviral substances is insufficient [24,25] [26]. Amalgamation of conventional knowledge and advance research techniques in the field of natural drug discovery imports a huge number of novel products from plants.
Due to less side effects and toxicity, it has been preferred by the common users. Not only that the cost-effectiveness of those natural products provides a value addition and also increased its accessibility [27]. Those products can be able to provide broad spectrum protective measures to numerous health hazards including viral infections. In case of the treatment of SARS-CoV-2, a formulation composed of 6.6% of each of the 15 plants, viz., Zingiber officinale (ginger), Adhatoda vasica (malabar nut), Piper longum (long pepper), Andrographis paniculata (bitterweed), Tragia involucrata (Indian stinging nettle), Syzygium aromaticum (clove), Terminalia chebula (chebulic myrobalan), Hygrophila auriculata (kokilaksha), Cyperus rotundus (java grass), Plectranthus amboinicus (Indian borage), Clerodendrum serratum (bharangi), Tinospora cordifolia (guduchi), Saussurea costus (costus), Sida acuta (teaweed), and Anacyclus pyrethrum (akarkara), has been recommended by the Ministry of AYUSH (Ayurveda, Unani, Yoga, and Naturopathy, Siddha, and Homoeopathy), Government of India [28]. The antiviral activity of many plant extracts and compounds of plant origin has been reported, which includes aqueous extracts of Cassine xylocarpa stem (Celastraceae), Maytenus cuzcoina root & bark (Celastraceae) [9], and crude methanolic extract of roots of four Lamiaceae members, viz., Thymus carmanicus (Thyme), Thymus vulgaris, Thymus kotschyanus, and Thymus daenensis [9,29], which are used to cure HIV infection; aqueous extract of Nerium oleander (kaner, Apocynaceae) is used against Poliovirus type 1 [30]; methanolic extract of Bryophyllum pinnatum (life plant, Crassulaceae), Mondiawhitei (white's ginger, Periplocaceae), Terminalia ivorensis (black afara, Combretaceae), Ageratum conyzoides (goat weed, Asteraceae) are used to cure echovirus [7]; aqueous and ethanol extracts of dried leaves of Andrographis paniculata (Acanthaceae) and Gynostemma pentaphyllum (poor man's ginseng, Cucurbitaceae) are used to prevent    There is not a single medicine, which is globally accepted for the treatment of dengue fever. However, extracts obtained from different plants are expected to lower the severity of dengue viruses by increasing the platelet count, which includes Carica papaya leaf extract that is used by the local people of North Eastern plain zone of India, Madhya Pradesh [33,56], whole plant of Andrographis paniculata (bitterweed), Alternanthera sessilis (sessile joyweed), Achyranthus aspera (prickly chaff flower), and Solanum xanthocarpum (yellow-fruit nightshade), and leaf and bark of Calotropis procera (calotrope) used in Bihar by traditional healers [32,33]. In some clinical trials, it has been confirmed that the administration of leaf extract of Carica papaya (papaya) on patients with dengue fever increases their plate-let count [33,45]. More specifically, as we are facing SARS-CoV-2 in present days, so it is required to know the name of the phytochemicals and their classes to check the intensity of this virus [57]. Table 2 represents a list of phytochemicals acting against various targets of SARS-CoV-2, where most of the phytochemical binds with spike protein ACE2 (like anthraquinone, emodin, rhein, and cinnamaldehyde) or inhibits main protease (M pro ) (like somniferine, isorientin, and gallocatechin-3-gallate) [57].

Kaempferia parviflora
Wall.  Inhibits viral replication [51] 7 BioMed Research International  [71]. Ignatov (2020) explained that high levels of potassium reported from Moringa oleifera helps the patient of COVID-19 to decrease the rate of infection caused by the SARS-CoV-2 virus. Different plants induce the production of numerous secondary metabolites such as phenolics, essential oils, glycosides, coumarins, alkaloids, terpenoids, and peptides mainly in response to microbial infections or to avoid herbivores and other organisms ( Figure 4). These metabolites have been accepted to play an essential role in boosting of the immune system and manifesting of antiviral potential [27,72]. There are several phytochemicals, which are found to be effective against different RNA viruses also; some of which are listed in Table 3.
Few recurrent and non-recurrent viral infections include human immunodeficiency virus type 1 (HIV-1) and 2 (HIV-2) [27], hepatitis C virus [6], dengue virus [33], and poliovirus. Furthermore, the control and prevention of different emerging viral diseases are imposing a great challenge to the society [88]. About 500,000 plant species are estimated to be present in the world. Interestingly, 10-15% among them are being used as drugs and 10% as a source of food [89]. However, the rapid extinction of some species leads to irretrievable loss of potential phytochemicals and also imposes another serious threat. The anti-RNA viral activity of active compounds from some medicinal plants have been reviewed and summarized in Table 4.
In case of the treatment of Ebola virus, many alkaloids are isolated from Atropa belladonna that gives a positive result and cure of Ebola haemorrhagic fever (Ebola HF) [100,101], whereas a sulfated polysaccharide, fucoidan isolated from Cladosiphon okamuranus, and sulphated polysaccharide (named kappa carrageenan) from whole plants of Gymnogongrus griffithsiae [94] worked against dengue virus type 2 (DENV-2). Cytarabine and matrine are two important active constituents found in the extract of Gillan's plants such as chuchaq and trshvash, respectively, whereas these compounds have the potentiality of better binding interaction with the receptors and show anti SARS-CoV-2 activity by inhibiting the initiation of viral infection [108].
There are three species of Tephrosia (viz. T. crassifolia, T. madrensis, and T. viridiflora), which can able to produce huge number of flavonoids and show antiviral activity against dengue virus. Among them, glabranine and 7-Omethyl-glabranine isolated from T. madrensis showed strong inhibitory effects on dengue virus replication in LLC-MK2 cells. Interestingly, moderate to low inhibitory effect was observed by methyl-hildgardtol A isolated from T. crassifolia. However, hildgardtol A of T. crassifolia and elongatine of T. viridiflora do not show any inhibitory effect on viral growth [94]. Honokiol (a lignin biphenol) derived from Magnolia tree has antiviral activity against serotype 2 dengue virus (DENV-2). This novel molecule can be able to interfere on the endocytosis of virus to reduce its doublestrand RNA, by abrogating the colocalization of DENV envelope proteins. Inhibitory activity of honokiol was proved by suppressing the replication of DENV-2 in baby hamster kidney (BHK) and human hepatocarcinoma Huh7 cells [9,47].
In the Chinese herbal medicine, extracts of Lonicerae japonicae (Japanese honeysuckle), Flos (ragged-robin), and Fructus forsythia (Lianqiao) are used together as an antimicrobial and anti-inflammatory agent. Interestingly, chitooligosaccharide in combination with herbal mixture can also act as an anti-influenza agent [64].

Major Challenges and Future Perspectives
Nowadays, the acceptance of herbal medicines is increasing for many reasons as it is less expensive, with less or null side effects and better patient tolerance. From the past to recent studies, it can be speculated that quality and safety study of plant-derived compounds with medicinal importance is required. Therefore, scientific, accurate, and informative studies are obligatory for any potent compound, which have isolated from plants [21]. About half million plants with medicinal properties are estimated to be present around the world. Surprisingly as well as grievously, most of them are yet not investigated [109]. It is required to give more attention to taxonomic characters and identification of wide range of medicinal plants throughout the world [21]. Various pharmacologically important peptides or proteins obtained from different medicinal plants can be introduced in the production of vaccines and therapeutics. The main challenge in this field of research is the extraction procedure. In this connection after establishment of a compound as a potent therapeutic agent, researchers have to increase its production in both in vitro and ex vitro conditions. This is also very challenging but can be resolved by using sophisticated techniques of recombinant DNA technology, elicitation, etc. [26]. Along with these, identification of biosynthetic pathway of that particular product and application of transgenic approach can be used as feasible way to enhance the production of that particular product. Different scientific approaches can also be taken to identify the newer plant-derived compounds with potential pharmacological activities. This can be considered as an emerging field of scientific research with global profit. Furthermore, it was already expected by [21] that within the year of 2020, the whole market in Asia-Pacific for herbal supplements reached more than US$115 billion and about more than that much turnover happened in 2021, i.e., US$140 billion [21]. It is also explained in the report of Asia Pacific Nutritional Supplements Market Size, Share & Trends Analysis Report with a report ID of GVR-4-68038-105-4 that it is expected that it will expand about 6.1% at a compound annual growth rate (CAGR) starting from 2022 to 2030. So, it is clear that the use of herbal supplements and ethnic medicines are increasing day by day. In the recent days, several RNA interference (RNAi)-based technologies and clustered regularly interspaced short palindromic repeats (CRISPR)-Cas technology have appeared to control viruses. It helps by either directly targeting the viral RNA or DNA or by inactivating plant host susceptibility gene [110]. The CRISPR-Cas system is a technology, which uses in diagnosis of various infectious RNA viruses like SARS-CoV-2 and HIV. This is undertaken as a potential technology for its accuracy both in therapeutic and molecular diagnosis [111].

Conclusion
Standing in the current circumstance of global pandemic of COVID-19, that causes huge damage of human health and also destroys economic backbone of a country, researchers are constantly trying to find out the way by which mankind will be safe from the fourth or fifth wave of virus onslaught. At this time of high crisis period, we all want something to eradicate or bypass the disease. We reviewed the plant extracts and/or extracted and identified active compounds from the medicinal plants that are useful for prevention of many RNA viral diseases, focusing on plant extracts and phytochemicals that have already reached in clinical trials and having highlighting properties to prevent the viruses. Along with disruption of global health, RNA viruses have immense potential to stop the economic growth of a country. However, for prevention of those diseases and to secure human populations, antiviral drug discovery is needed. Till date, many traditional medicinal plants were reported to have strong antiviral activities. Medicinal plants contain active compounds like alkaloids, saponins, flavonoids, tannins, polysaccharides, proteins, and peptides. These 17 BioMed Research International compounds can help in the prevention of viral diseases by blocking the virus entry and/or via the inhibition of viral replication at the different stages by regulating the enzyme actions as well and specially without having any known side effects. Finally, the improvement of newly discovered medicinal plant products is vital and required for controlling the constant threats of deadly contagious RNA viruses. This review enlightens some way with lesser hazard to combat in this epidemic era of RNA virus like COVID-19. It is necessary to further examine by highlighting drug delivery system of antiviral phytochemical to reach successfully in their intended site of action.

Data Availability
The data used to support the findings of this study are included within the article.

Ethical Approval
This article does not contain any studies with human participants or animals.

Conflicts of Interest
The authors report no financial or any other conflicts of interest in this work.

Authors' Contributions
Anamika Paul, Krishnendu Acharya, and Ram Prasad are responsible for the conceptualization; Nilanjan Chakraborty, Anuj Ranjan, and Abhishek Chauhan for the investigation, data collection, and resource; Anamika Paul and Anik Sarkar for the formal analysis and writing-original draft preparation; Nilanjan Chakraborty for the writing-review and editing; Shilpi Srivastava, Akhilesh kumar Singh, Iqra Mubeen, and Ashutosh Kumar Rai for the data analysis and editing; Ram Prasad for the writing, review, and editing; and Krishnendu Acharya for the supervision. All authors read the final manuscript. Anamika Paul and Nilanjan Chakraborty contributed equally to this work. All authors agree mutually to the publication of this work.